1. Field of the Invention
The present invention relates to floating roof liquid storage tanks of the type that are extensively used to store liquid hydrocarbon products such as crude oil, gasoline, and the like.
2. Description of the Prior Art
Oil refineries and storage terminals utilize floating roof tanks for the storage of hydrocarbon stocks which have a higher vapour pressure than products which can be stored in cone roof tanks. Typical of such products are gasoline, naphthas and crude oil.
The filling and emptying cycle of such tanks is between a normal minimum to a normal maximum gauge (or depth) which typically is approximately 2 meters to approximately 14 meters respectively. The minimum gauge elevation is determined by the need to keep the underside of the roof clear of any projections into the tank (e.g. tank heaters, mixers, suction/rundown lines) and the requirement to provide sufficient head for pumping equipment connected to the tank. As all working movements in the tank are above the minimum gauge, the volume in the tank at minimum gauge, (or heel) is a static inventory which represents a high cost.
Typically a floating roof tank is at least approximately 20 meters in diameter. Thus a cylindrical heel 30 meters in diameter and 2 meters thick contains many barrels of valuable liquid. This liquid must be purchased but cannot be sold as it cannot be extracted from the tank whilst the tank remains in use.
It is known from U.S. Pat. Nos. 2,924,350; 2,947,437 and 3,167,203 to have a water/stored liquid interface which utilizes the fact that most hydrocarbons will float on water. However the arrangements of those patents are not directed to reducing heel inventory, and suffer from the severe disadvantage that maintanence of the tank wall, because of corrosion, must be carried out under water.